I've used them several times and used the brites and metallics most of the time. I've used them with stencils on fabric, used them to do rubbings of leaves to make wall hangings, etc.
I think that the older kids would be able to use them but they're pretty expensive so not sure if it would be cost effective for use by younger kids. They are also a little messy - taking the dried layer off when you use it, etc, and since it's really oil paint, cleanup is more difficult than other types of paint.
Freezer paper works great as stencils -
As much as I love using them, I think you'd be better off finding something cleaner and cheaper to use with the kids.